In radio frequency (RF) receivers containing integrated active filter stages such as zero intermediate frequency receivers (ZIF receiver), automatic gain control (AGC) circuit is typically employed. The function of the AGC circuit is to control the signal level to the active filter stages in the receiver so the signal is not driven in to limit (clipping) conditions. Automatic gain control circuits attempt to keep the output level of the receiver constant regardless of received input signal strength. Typically, the receiver gain is regulated in inverse proportion to the signal strength of the received signal.
In order to protect from signal over-drive conditions in a multi-stage active filter arrangement such as found in a ZIF receiver, the stage under gain control is usually selected as one of the first stages in the receiver path. This less than optimum implementation of an AGC system results in a decrease in the maximum signal-to-noise ratio whenever large input signals are present at the receiver's front-end. This decrease in signal to noise ratio is caused by the fact that the noise floor does not decrease as much as the signal level when AGC is applied. In other words, the decrease in signal to noise ratio is caused by a reduction in system takeover by tile AGC circuit.
In other prior art AGC systems gain reduction has been applied to multiple amplifier stages simultaneously in order to control multiple stages of a receiver to avoid signal distortion. A good example of such a system can be found in U.S. Pat. No. 4,850,038 entitled "Frequency converter" by Shibata et al. Unfortunately, under strong input signal conditions such as when a pair of transceivers are communicating with each other at a close distance, even prior art systems that rely on multiple AGC controlled stages have problems with signal distortion due to signal clipping. A need thus exists for a communication receiver AGC system which can provide for improved maximum signal to noise ratios even under large received input signal conditions.